Fingerprint sensing technology is increasingly recognized as a reliable and convenient way to identify individuals and to verify individual identity. Fingerprints, like various other biometric characteristics, are the distinctive, measurable characteristics used to label and describe an individual. The applications for fingerprints sensors are plentiful. For example, fingerprint sensors may be used to provide access control in stationary applications, such as security checkpoints, door locks. Fingerprint sensors may also be used to provide access control in portable applications, such as portable computers, smart phones, information appliances, and data storage devices. Recently, fingerprint authentication technologies are further developed to be used in a wider range, such as payment authorization on a portable device, such as a credit card. Accordingly, some applications, particularly portable applications, may require fingerprint sensing devices to be compact, highly reliable, and inexpensive.
Various fingerprint sensing methods, techniques, and devices have been in use or proposed. For example, optical and capacitive fingerprint sensing devices are currently on the market or under development. Optical fingerprint sensors utilize reflected light from a fingertip to capture a digital image. On the other hand, capacitive fingerprint sensors utilize distribution of electrical field which is affected by the capacitance between the fingertip and the sensor to generate a fingerprint image.
Generally, there are two types of capacitive fingerprint sensing techniques in the market now: passive and active. Both types of capacitive techniques utilize the principle of capacitance to generate fingerprint images. Passive capacitive technique, such as the sensor disclosed in U.S. Pat. No. 5,325,442 by inventor Alan G. Knapp and U.S. Pat. No. 6,016,355 by inventor Alexander George Dickinson, utilizes an array of sensing units (each sensing unit contains a metal plate as a sensing electrode), to apply an electrical current to the capacitance formed between the metal plates and the fingertip. The method obtains the capacitance differences between the sensing units by measuring the different charging or discharge currents, and further using the capacitance differences to distinguish ridges from valleys. Others, such as U.S. Pat. No. 7,663,380 by inventor Kai Lan Chuang and U.S. Pat. No. 7,099,497 by inventor Bruce C. S. Chou, improve the idea of Knapp by utilizing pre-charged capacitors as a source of the charging current/voltage to improve the sensitivity of the sensing units. However, the signal strength and image quality decline significantly as the thickness of the protective layer increases.
Active capacitive technique, such as the sensor disclosed in U.S. Pat. No. 5,940,526 by Dale R. Setlak, is similar to passive technique but requires excitation to the epidermal skin layer of the sensed fingertip by applying different voltages to the sensed fingertip via a metal ring or strip. Active capacitive technique may render a better image quality while the packaging structure meets the need of the sensor protection by increasing the thickness of the cover material. However, there is a major disadvantage in active capacitive technique. The excitation voltage applied via the metal ring is significantly reduced when a low impedance path that leads to ground exists between the metal ring and human body. This problem causes the system to be unreliable, particularly for the portable devices with chassis connected to ground. The workarounds of this problem may either require a sophisticated ground isolation structure, such as the sensor disclosed in U.S. Pat. No. 9,152,841 by Frank Robert Riedijk, or much higher excitation voltage level which also increases complexity and cost.
In order to overcome the aforementioned drawbacks of passive and active capacitive fingerprint sensing techniques, a capacitive fingerprint sensing device that can achieve a high-quality fingerprint representation even if a thick protective layer is used without applying excitation voltage to the fingertip is highly desired.